The present invention relates to the packaging of items under certain conditions, including pressure greater than ambient pressure, where such conditions can be beneficial for purposes such as safeguarding or preserving the item.
The invention may be applicable, for example, to preserving products as described in U.S. Pat. No. 5,093,080 to Elbert G. Keller, in U.S. Pat. No. 2,994,424 to Myer S. Selby et al, or in United Kingdom Patent 710,979 to Bezalel Niedzwiedz. The aforecited Keller patent, for example, describes ways to preserve products which have a cellular respiration and an internal vapor pressure. As is disclosed in the Keller patent, such respiratory products have a partial vapor pressure which is greater than the partial vapor pressure in the surrounding atmosphere, forcing moisture and nutrients to leave the product and resulting in drying and early death or deterioration of the product. In accordance with the patent, a positive pressure gradient with respect to the internal vapor pressure of the product is provided in an enclosed area surrounding the product to negate the outward diffusion of moisture and nutrients of the product.
It will be appreciated that the aforementioned Keller patent provides a way to preserve products without certain of the limitations of the prior art. In addition, it will be appreciated that positive pressure within a container can provide the added benefit of preventing contaminates from readily entering the package. Moreover, internal pressure permits packaging which resists damage to the contents of the package because packaging can be constructed so that the internal pressure adds structural strength to the exterior of the package.
Further consideration of the problem of storing and preserving living organisms, particularly after harvest, has led to the belief that there are a variety of other conditions, especially as they relate to packaging, which affect the preservation of such products. Thus, for example, certain fresh foods stored in such a way that they are completely deprived of oxygen or there is insufficient oxygen for the storage period may cause food poisoning as a result of the onset of an anaerobic condition.
Increasing the pressure in the container will increase the amount of oxygen for a given volume of air space and will thus effectively extend the storage life. This will permit a more efficient use of containers or packages and has the added benefit of permitting the product to be cooled more rapidly since the coefficient of heat transfer by conduction increases with an increase in pressure.
However, there are limits to the extent to which pressure can be increased without adversely affecting some stored products. For example, the cellular structure of strawberries may be damaged by high pressures whereas meat may withstand such pressures without damage. Thus, with a product like meat, the effective free air space may be acceptable despite little remaining volume in the container or package if the pressure is maintained at a high level with respect to ambient pressure.
Another factor which has an effect on the free air space required to avoid an anaerobic condition in a container is the respiration rate of the product in the container. Different products have different respiration rates, and the respiration rate of a particular product also will vary as a function of temperature. Thus, the oxygen in the container will be used at different rates depending on the nature of the product and its temperature. Moreover, the length of time the product is to be preserved in the container will be a factor in determining the amount of free air space needed in the container to avoid an anaerobic condition.
In addition to conditions relating to free air space and the avoidance of an anaerobic condition in the container, other steps may be taken to extend the life of respiratory products. As is noted in the aforementioned Keller patent, pressurization may be used independently or in conjunction with depressed temperature, to preserve such products.
It will be appreciated that under positive pressure the freezing temperature (phase change temperature) for any product is lower than at normal atmospheric pressure. Phase change temperature is the temperature at which the liquid components change from a liquid to a solid and, in the case of water, expands in volume. Such expansion can damage cell structure.
At depressed temperatures the respiration rate of the product is lowered. If the temperature is depressed below the normal freezing temperature of the product at atmospheric pressure but above the freezing temperature of the product under positive pressure, this even lower temperature further depresses the respiration rate and further contributes to the preservation of respiratory products without a change of state from liquid to solid and cellular damage to the product. Thus, using pressure in accordance with the aforementioned patent in conjunction with temperatures below the normal freezing point of the product but insufficiently low to freeze the pressurized product itself will provide beneficial effects while avoiding cellular deterioration of the product.
One problem in conventional systems for pressurizing a product relates to opening a pressurized container. In the aforementioned patents, there is no provision for reducing the pressure of the container before opening the container. Thus, if the pressurized container holds any type of liquid, there is a great likelihood that it will be sprayed on the user when the pressurized package is opened. Accordingly, it would be beneficial to have a package which could be depressurized before being opened.